The invention relates to a one-step process for the preparation of liquid-crystalline monomers containing acryloyl groups.
In general, the production of three-dimensional networks having a desired optically anisotropic property profile requires crosslinkable liquid-crystalline monomers in which the mesogenic units and the crosslinkable groups are separated front one another by spacer units. The incorporation of the spacers increases the mobility of the acrylic groups and is vital for achieving a sufficiently high degree of crosslinking. In addition, the liquid-crystalline phase of the monomers can be positively affected by the choice of suitable spacer lengths.
Owing to the necessity for a spacer, however, the desired monomers cannot be obtained by simple (meth)acrylation of the mesogenic alcohols, which are usually readily accessible. In some cases, significantly more complex synthetic routes must be followed here. It is in many cases favorable for the spacer to be bonded to the mesogenic unit via an ether bond. The corresponding compounds are particularly advantageous owing to their relatively good synthetic accessibility, their chemical stability, but in particular owing to their frequently very advantageous liquid-crystalline property profile.
The literature therefore describes numerous synthetic routes which in principle are suitable for the synthesis of crosslinkable mesogens containing corresponding spacers. However, these synthetic routes are without exception afflicted with specific disadvantages or limitations. Thus, for example, WO 96/24647 and WO 96/23036 describe processes in which a mesogenic diol is first reacted with an xcfx89-haloalcohol. The resultant intermediate is subsequently esterified using (meth)acryloyl chloride to give the finished product. There are numerous problems associated with this process. First, the alcohols produced as intermediates generally have poor solubility properties, making not only their preparation, but also their isolation and purification, very complex. Second, the xcfx89-haloalcohols required as starting materials are relatively expensive compounds which frequently cannot be produced on an industrial scale. This synthetic route is therefore disadvantageous for commercial production.
In another process, described, for example, in WO 98/47979 and EP 0 648 827, the starting materials are xcfx89-halo(meth)acryloylalkanes, which can be reacted with a mesogenic monol or diol in a single step to give the desired end product. The labor-intensive isolation of a mesogenic intermediate containing hydroxylated spacer units is not necessary here. However, a disadvantage of this process is the preparation of the required xcfx89-halo(meth)acryloylalkanes starting materials, since these compounds have a strong tendency toward undesired polymerization, and are therefore virtually impossible to handle without a stabilizer. The tendency toward polymerization is particularly pronounced in the case of the acryloyl compounds, and thus their synthesis and purification is only possible with difficulty in virtually all cases. In addition, scale-up of such reactions to an industrial scale requires complex safety precautions.
Finally, the literature also describes a number of preparation processes which follow a completely different strategy. However, it is common to all these that they can without exception only be achieved by multistep, and therefore very complex, synthetic routes.
The present invention has the object of overcoming the disadvantages of the known synthetic routes and of providing a process by means of which crosslinkable mesogens containing spacer-bonded acrylic groups can be prepared simply and in an acceptable manner with respect to the reaction and safety. This process can prepare the desired crosslinkable mesogens economically and in a single step.
The invention relates to a one-step process for the preparation of acryloyl group-containing liquid-crystalline monomers of the general formula (1)
(Zxe2x80x94Y1xe2x80x94A2xe2x80x94Y2xe2x80x94)mM(xe2x80x94Oxe2x80x94A1-acrylate)nxe2x80x83xe2x80x83(1)
in which mesogenic alcohols of the general formula (2)
(Zxe2x80x94Y1xe2x80x94A2xe2x80x94Y2xe2x80x94)mM(OH)nxe2x80x83xe2x80x83(2)
are reacted with esters of 3-chloropropionic acid of the general formula (3)
ClPrxe2x80x94A1xe2x80x94Xxe2x80x83xe2x80x83(3)
with elimination of HCl, where
acrylate is an acrylate radical,
ClPr is a 3-chloropropionate radical,
A1 are identical or different alkyl chain spacers having 2-20 carbon atoms, in which the carbon chain may be interrupted by non-adjacent ether oxygen atoms, thioether sulfur atoms, or imino groups,
A2 are radicals A1 or single chemical bonds,
M is a mesogenic group,
X is a leaving group,
Z are alkyl radicals or crosslinkable groups,
Y1 and Y2, independently of one another, are a single chemical bond, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94O, xe2x80x94COxe2x80x94NR1xe2x80x94, xe2x80x94NR1xe2x80x94COxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94NR1xe2x80x94, xe2x80x94NR1xe2x80x94COxe2x80x94Oxe2x80x94 or xe2x80x94NR1xe2x80x94COxe2x80x94NR1xe2x80x94,
R1 is hydrogen or a C1-C4-alkyl radical,
n has a value of 1, 2, 3 or 4, and
m has a value of 0, 1, 2 or 3.
The process according to the invention starts from a mesogen of the general formula (2) containing one or more free hydroxyl groups. The invention is based on the discovery that, in a single synthetic step, the ether bond between the mesogenic alcohol of the general formula (2) and the 3-chloropropionate of the general formula (3) can be formed with elimination of the leaving group X and simultaneously the acryloyl group can be liberated from the chloropropionate group by elimination of an HCl molecule.
The process according to the invention is highly suitable for the preparation of mesogens containing 1 to 4 spacer-bonded acrylic groups, where n has a value of from 1 to 4, in particular 2. The desired mesogen of the general formula (1) may be prepared in only a single synthetic step from readily accessible starting materials. Further, the process according to the invention allows a synthesis concept scheme in which the acrylic acid functionality of the 3-chloropropionate precursors of the general formula (2) are masked. The demasking is delayed until during the final reaction step of the entire mesogen synthesis, the described etherification of a mesogenic alcohol. Thus, unpolymerizable starting materials are exclusively employed in the synthesis sequence for the preparation of acrylated mesogens of the general formula (1), and in addition no polymerizable intermediates are formed. The performance of the individual synthetic steps, but in particular the handling of the intermediates, is thus significantly simplified.
A further advantage of the process according to the invention consists in the fact that the esters of the required 3-chloropropionic acid starting materials of the general formula (3) can easily be prepared by reaction of suitable alcohols or cyclic ethers with 3-chloropropionyl chloride, a chemical produced on a large industrial scale. Direct esterification of the free 3-chloropropionic acid, for example by azeotropic esterification using the targeted alcohol derivatives, is also possible. The reaction according to the invention is thus also extremely valuable for industrial applications.
The process according to the invention is suitable for the preparation of any desired liquid-crystalline compounds containing one or more spacer-bonded acrylic groups of the general formula (1). It is preferably employed for the preparation of liquid-crystalline bisacrylates of the general formula (1) in which m is 0 and n is 2 by reaction of a mesogenic diol of the general formula (2) in which m is 0 and n is 2, with 2 equivalents of a compound conforming to the general formula (3). Use of mixtures of a plurality of compounds conforming to the general formula (3) also allows the preparation of stoichiometric mixtures of symmetrical and asymmetrical bisacrylates. Preferably, only one compound of the general formula (3) is employed.
Furthermore, the process according to the invention can also be employed for the conversion of liquid-crystalline alcohols which already contain crosslinkable groups of the general formula (2) in which m is greater than 0 and Z is a crosslinkable group. The process can also be utilized for the preparation of mesogens containing two different crosslinkable groups and/or spacers, in which case, in the general formulae (1) and (2), m is 1, n is 1 and Z is a crosslinkable group.
Suitable radicals M are all known mesogenic groups. M preferably conforms to the general formula (4)
(xe2x80x94Txe2x80x94Y3)rxe2x80x94Txe2x80x94xe2x80x83xe2x80x83(4)
in which
T are divalent saturated or unsaturated, isocyclic or heterocyclic, substituted or unsubstituted hydrocarbon radicals having 5-20 carbon atoms, preference being given to benzyl, cyclohexyl and naphthyl radicals,
Y3 is a single chemical bond, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94NR2xe2x80x94, xe2x80x94NR2COxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94NR2xe2x80x94, xe2x80x94NR2xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94NR2xe2x80x94COxe2x80x94NR2xe2x80x94, xe2x80x94CH2xe2x80x94Oxe2x80x94,xe2x80x94Oxe2x80x94CH2xe2x80x94, xe2x80x94CHxe2x95x90Nxe2x80x94, xe2x80x94Nxe2x95x90CHxe2x80x94 or xe2x80x94Nxe2x95x90Nxe2x80x94,
R2 is hydrogen or a C1-C4-alkyl radical, and
r has a value of 0, 1, 2, 3 or 4, particularly preferably 1 or 2.
In the case where r is greater than 0, the radicals T may be identical to or different from one another, as may the bridging radicals Y3. The radicals T can also be more highly substituted, for example by C1-C4-alkyl groups, fluorine, chlorine, bromine, cyano, hydroxyl, (alkyl)amino or even nitro groups. Particularly preferred radicals T are benzyl and cyclohexyl groups, which may, if desired, carry further substituents.
Particular preference is given to mesogenic groups M which conform to the structural formula (5) or (6) 
where each ring can carry one or more identical or different substituents.
Particularly preferred mesogenic diols of the general formula (1) conform to the general formula (7) or (8) 
These compounds are readily accessible via known synthetic routes from hydroquinone and p-hydroxybenzoic acid or from substituted derivatives of these compounds, as described, for example, in Polym. Prep. 30 (2), 462-3, 1989.
The alkyl radicals Z in the general formulae (1) and (2) are preferably C1-C4-alkyl radicals. The crosslinkable groups Z can be all groups known to those skilled in the art. Preferred groups Z are alkenyl groups, such as vinyl, 2-methylvinyl, and allyl groups, and alkynyl groups and epoxy groups.
The leaving group X in the general formula (3) can be any group known to the person skilled in the art. Preference is given to halogens and sulfonic acid derivatives, where the latter, such as, for example, triflate of the formula (10), can also contain fluorine-containing radicals. The formulae (9)-(13) 
are non-limiting examples of preferred sulfonic acid radicals of this type.
Particularly preferred leaving groups X are chlorine and bromine leaving groups, in which case it is advantageous to carry out the etherification according to the invention in the presence of catalytic amounts of potassium iodide.
For the preparation of suitable compounds of the general formula (3), an extremely extensive repertoire of different processes is available to the chemist. The simplest reaction schematically starts from an xcfx89-haloalcohol, which is reacted with 3-chloropropionyl chloride to give the corresponding ester (X is halogen). Owing to the restricted availability of inexpensive xcfx89-haloalcohols, however, this route is only in exceptional cases of industrial interest.
It is generally preferably here to start with an alkanediol. The alkanediols can be reacted, for example, with 3-chloropropionyl chloride under suitable conditions to give the corresponding 3-chloropropionic monoesters. Esterification of the still-free hydroxyl function in the latter using sulfonyl chlorides enables the simple preparation of activated sulfonic acid derivatives of the general formula (3) in which X is a sulfonic acid derivative.
It is also possible to replace the free hydroxyl group in the above 3-chloropropionic monoester by halogen atoms, for example by reaction with thionyl chloride. The compounds which can be prepared in this way are likewise suitable as starling material of the general formula (3) in which X is a halogen atom.
A particularly efficient and elegant reaction for the preparation of the compound of the general formula (3) is the ring opening of cyclic ethers, such as tetrahydrofuran, by means of 3-chloropropionyl chloride. 4-chlorobutyl 3-chloropropionate is prepared in this manner in only a single, very simple reaction step from extremely inexpensive starting materials.
In the examples below, unless stated otherwise in each case, all amounts and percentages are based on the weight, all pressures are 0.10 Mpa (abs.) and all temperatures are 20xc2x0 C.